Ogretmen olunca sorunun gelisinden anliyor ne istendigini: You asked for a "simple explanation" which suggests:
1) You want a no-nonsense overview without jargon
2) You want something that will help you learn from the beginning
3) You have discovered that no two people ever answer the question the same way, and it's confusing. 4) That's the reason you are here asking for a simple explanation, yes?